Encoding cookies so they cannot be spoofed or read etc

I am writing a PHP application. I want to store user login information in cookies so user's dont have to log in on every visit.

I want to encode them or obfuscate them so that they cannot be read or tampered with.

What is the best way to do this?

Update:

I am not going to be storing passwords in the cookies, simply a user ID so that I know who they are, but I want this to be encoded or encrypted so no one can spoof other users